@karajorma
The on-point nature of your reply is appreciated.
And i agree, the occurrence is quite interesting. It was an accidental find [cue recap]: i edited "Their Finest Hour" to include Zeus spacecraft, and was using the Maxim primary plus a Subach (as backup, versus fighters). At some point i stopped engaging fighters altogether, so the Subach became unnecessary, and thus i left one primary empty at loadout. Later in the mission i was then bewildered to see a Prometheus-R in the empty spot; for a moment i doubted my sanity, but a quick test pointed to the support ship adding the Prometheus during rearm.
i did perform searches both here and on Mantis before posting, to verify whether or not this was known, and didn't come across any mention of it. Since i had no idea whether or not it is a bug or feature, i simply described it as "interesting". (Personally i find it slightly problematic, because if i deliberately go with only one primary, i don't want another added in mid-mission.)
Now that several people have responded, i'm a little surprised... some say the rearm-primary-thing was known, yet some appear to have been unaware. Very strange. i wonder: is it FS-Open specific? Does it happen in retail behaviour as well?